+1 on the electric heater. Late in the season, after our last trip but before I winterize, I keep an eye on the weather forecast. If they say it's going to be below freezing, I open the cabinets that have plumbing in them and turn on an electric heater. It's enough to keep everything from freezing.
Geoff & Jill
& Sierra, the little white monster
2013 Ford F-150 XTR SC Ecoboost
2015 Jayco Jay Flight 24FBS